Bug of the Month May 2020
Pea Leaf Weevil
by Asha Wijerathna,
The Pea leaf weevil, (Sitona lineatus) is an insect that feeds on leguminaceous plants. Adult pea leaf weevils feed on a variety of legumes such as alfalfa, soybean, lupin and lentil to a lesser extent, but is a major pest of field pea and faba bean. Pea leaf weevils can cause important economic damage in field pea and faba bean and better management strategies are necessary to control this insect pest. Pea leaf weevil adults overwinter in field margins and tree shelters, particularly those containing perennial legumes. In spring, adults emerge from overwintering sites when the temperature is above 12.5°C and disperse in search of host plants to eat. Adults feed on the foliage of a host plant making “U” shape notches along leaf margins.
Females deposit eggs in soil cavities close to the host plant and emerging larvae feed on Rhizobium leguminosarum bacteria associated with root nodules of host plants. Larval damage alone or damage by both larvae and adults can cause yield losses in field peas and faba bean. Adult pea leaf weevils can damage developing seedlings under high pest densities.
The current threshold for pea leaf weevil on field pea is 30% of seedlings with clamp leaf damage at the 3rd node stage of the plants. You can determine the threshold by counting the number of plants that have pea leaf weevil adult “U” shaped notches on the terminal clam leaves at the 3rd node stage of the plants. For faba bean the threshold is lower than the field peas, 15% of the seedlings with terminal leaf damage. This means that yield losses occur when 2 out of 10 plants have the clam leaf damage at the 3rd node stage.
Several research studies have shown that insecticides coated on the seed are more effective than foliar insecticides. This is because the weevils may have laid enough eggs when the insecticides are sprayed or more weevils may move into the fields after the insecticide is applied. Natural enemies in the crop field are likely important in controlling pea leaf weevil. Smaller carabid beetles like Bembidion sp. are known to be predatory on pea leaf weevil eggs. Larger species of carabids such as Pterostichus melanarius have been observed feeding on adult weevils.